1. Field of the Invention
The present invention generally relates to implantable medical apparatus and more specifically relates to implantable apparatus for electrical sensing or stimulation of muscle tissue.
2. Description of the Prior Art
The earliest users of transvenous leads for artificial intracardiac pacing produced unacceptable dislodgement rates which yielded substantially increased chronic pacing thresholds. In response thereto, a number of fixation devices have been employed. "Active fixation" which pierces tissue is taught by Bisping in U.S. Pat. No. 4,106,512. Bisping uses a helical fixation coil which is rotated and advanced by rotating the connector pin. In this way the distal end of the pacing lead is screwed into the endothelial tissue and thereby secured.
A second fixation technique is that of "passive fixation" which does not pierce tissue as taught by Citron et al. in U.S. Pat. No. 3,902,501. The use of tines for passive fixation is extremely common in the art.
The leads taught by Bisping and Citron et al. are intended for use in sensing and pacing within a single chamber. However, the current trend has been toward physiological pacing which involves sensing and pacing within both the right atrium and right ventricle. This may be accomplished by passing two separate leads, one for each chamber, using two different veins or even a single vein. A preferred method, however, is the use of a single pass lead which enables the implantation of atrial and ventricular electrodes from a single lead body.
U.S. Pat. No. 4,057,067 issued to Lajos teaches an early single pass lead. The primary difficulty with the Lajos lead is that the atrial and ventricular electrodes are located at a fixed distance from the bifurcation point. This provides difficulty in securing optimal positioning of both electrodes for implantation in hearts of varying size.
The co-pending Smyth et al. and Stokes applications referenced above the assigned to the assignee of the present invention describe techniques for overcoming the problem of varying heart size. Smyth et al teach a "slider" method whereby the interelectrode spacing is adjustable at the time of implant. This approach is effective, but requires establishing a tight seal at the point of bifurcation and cutting off the excess conductor length in the operating room.
Stokes teaches a method of achieving a similar result by letting the excess conductor length be stored chronically within the atrium. This technique requires storage of a great deal of excess conductor with a possible diminishing of hemodynamic efficiency.